Malaria remains one of the most important parasitic diseases in the tropics. In endemic zones, malaria is an integral part of the environment and comprises one of the most powerful deterrents to the development of vast geographical areas. In such zones, all inhabitants are infected from birth to death, several times a year (up to 1,000 times in Congo), and only survive because of defense mechanisms acquired during the first years of life. It is during these early years, especially between the age of 6 months and 2 years, that malaria causes mortality, at a rate difficult to estimate.
Today, nearly two billion human beings live in endemic areas, often in highly unfavorable conditions, and are exposed to the risk of malaria, suffering its morbid or even fatal effects, often without health care. Protecting these risk groups, which account for more than one-third of the earth's population, is a major public health challenge. Several million cases of malaria are registered each year. Most, including the most severe often fatal forms of the disease, are caused by Plasmodium falciparum (80% of the cases worldwide). Developed countries are not spared: the number of imported cases increases steadily due to the progression of international transportation.
Drug-resistant Plasmodium falciparum is now the greatest threat to the fight against malaria. Resistance appeared in the early sixties in Thailand and in Columbia, then spread, reaching Africa in 1978. Movements in populations have also played a role in the development of resistance and chloroquine-resistant Plasmodium falciparum is now widespread throughout the world. Resistance to the second line sulfamide/pyrimethamine combination is already widespread in chloroquine-resistant areas of Southeast Asia and South America. The current emergence of multidrug resistant strains, unresponsive to any of the available antimalaria drugs, is a major threat.
Among the currently available methods used to fight against malaria, the fight against larvae and the reduction of the source remains to be analyzed in terms of reduced case incidence. These methods do not appear to have a decisive effect on malaria. Methods based on treating homes with insecticides have several inconveniences (resistance, poor acceptance by the population, high cost, no effect in the savannah environment) while personal protection using pyrethrinoid impregnated nets is known to be effective, but subject to many limitations.
A polyvalent vaccine, fully active against the different forms of the parasite and all the types of malaria, has constantly been postponed and now appears to be far off (Walsh J, Science 1987, 235, 1319, Butcher, Parisitologie, 1989, 98, 315). For many years to come, chemotherapy will remain the necessary method for populations living in endemic zones.
More than 250,000 compounds tested in a major research effort undertaken in 1963 at the Walter Reed Center in Washington D.C. led to market approval for mefloquine (Lariam.RTM.) in 1985. Resistance to this new antimalaria drug has however been induced experimentally and cases of resistance have been reported. Of even greater concern is the cross-resistance demonstrated between mefloquine and other amino-alcohols such as quinine or new drugs currently in the experimental phase such as halofantrine marketed in 1989 (Halfan.RTM.) (J. Rieckmann, Ann. Rev. Med. 1983, 34, 321-335; D. Warhust, Drugs, 1987; 33: 50-65; Struchler, Parasitol Today 1989, 5:39-40).
Babesia, which belong to a hematozoan class comparable with Plasmodium, are particularly important animal parasites. Babesia and Plasmodium are very similar, but Babesia usually causes animal disease, mainly infecting cattle and dogs. The principal causal species are Babesia bovis, Babesia cani, Babesia gibsoni, Babesia divergens, and Babesia gibemina. Babesia equi is specifically implicated in equine disease.
The inventors have evidenced a new class of compounds with strong antiprotozoal activity, particularly antimalaria and antibabesiosis activity. In addition, this antimalaria activity exhibits a novel mechanism and could thus avoid much feared drug resistance in this therapeutic class. The concept of these compounds was guided by the demonstration that phospholipid metabolism in the malaria-infected red cell is abundant and specific. Asexual parasite proliferation occurring within the erythrocyte (the parasite phase associated with clinical signs of the disease) is accompanied by substantial phospholipid (PL) neosynthesis required for the biogenesis of Plasmodium membranes. The intraerythrocyte phospholipid level increases after malarial infection, rising to 500% when the parasite has reached its mature state.
Consequently, there is an excess of phospholipid biosynthetic metabolism in the erythrocyte after Plasmodium infection. In host mammals, mature erythrocytes are totally devoid of PL biosynthesis.
The different biosynthesis pathways of phosphatidylethanolamines (PE) and phosphatidylcholine (PC) are schematically represented in FIG. 1. The present invention utilizes the observation that the parasite's development is blocked by substances interfering with this metabolism at doses quite below those interfering with healthy cells. Certain commercial quaternary ammoniums, with other known therapeutic activities or not, inhibit the development of Plasmodium falciparum but at doses incompatible with use as drugs because of side effects, notably on the cholinergic system. These quaternary salts are trimethylalkylammoniums long recognized for their effect on surface tension or their smooth muscle relaxing properties (see decamethonium, Procuran.RTM.) when given via parenteral administration. ISOMAA (Acta Pharmacol, Toxicol 45 (5) 1979 and Biochem. Pharmacol 28 (7) 975-980) studied the action of quaternary alkyltrimethylammoniums on erythrocyte membranes in the rat and noted a protective effect on the membrane at low concentrations and a hemolytic effect at higher doses. These long-chain quaternary trimethylammoniums appear to induce changes in cell surfaces at relatively high concentrations (ISOMAA Acta Pharmacol Toxicol. 44(1) 1979 36/42). The mechanism of action, modifying the plasma membrane double lipid layer, would involve a surface tension mechanism effective at relatively high concentrations.
The series of long straight chain bis trimethylalkylammoniums have been studied, notably for their depolarizing activities on diverse cell models (Kratskin I, Gen. Pharmacol. 1980, 11(1) 119-26 and Skylarov Dokl. Akad; Nauk SSSR 1988 303 (3) 760-3). The only therapeutic activities are cholinomimetic or cholinolytic effects noted by Danilov AF (Acta Physiol; Acta Sci. Hung 1974 45 (3-4) 271-80). It is noted that all these agents affecting surface tension carry trimethylammoniums on both ends of a hydrocarbon chain generally composed of 12 methylene moities. The nitrogen atom thus carries identical, relatively small, substituents on a carbon chain usually less than 12 atoms long.